Microphone arrays including at least two microphones receive sound, convert the sound into sound signals, and process the sound signals to set a sound reception range in a direction of a source of target sound or control directivity. As a result, such a microphone array may perform noise suppression or target sound emphasis.
In order to improve an S/N (signal-to-noise) ratio, microphone array apparatuses disclosed in “Microphone Array”, The Journal of the Acoustical Society of Japan, Vol. 51, No. 5, pp. 384-414, 1995 control directivity and perform subtraction processing or addition processing on the basis of the time difference between signals received by a plurality of microphones. As a result, it is possible to suppress unnecessary noise included in a sound wave transmitted from a sound suppression direction or a direction different from a target sound reception direction and emphasize target sound included in a sound wave transmitted from a sound emphasis direction or the target sound reception direction.
In a speech recognition apparatus disclosed in Japanese Laid-open Patent Publication No. 58-181099, a conversion unit includes at least two speech input units for converting sound into an electric signal, a first speech input unit and a second speech input unit. The first and second speech input units are spaced at predetermined intervals near a speaker. A first filter extracts a speech signal having a predetermined frequency band component from a speech input signal output from the first speech input unit. A second filter extracts a speech signal having the predetermined frequency band component from a speech input signal output from the second speech input unit. A correlation computation unit computes the correlation between the speech signals extracted by the first and second filters. A speech determination unit determines whether a speech signal output from the conversion unit is a signal based on sound output from the speaker or a signal based on noise on the basis of a result of computation performed by the correlation computation unit.
In an apparatus disclosed in Japanese Laid-open Patent Publication No. 11-298988 for controlling a directivity characteristic of a microphone disposed in a speech recognition apparatus used in a vehicle, a plurality of microphones for receiving a plane sound wave are arranged in a line at regular intervals. A microphone circuit processes signals output from these microphones and controls the directivity characteristics of these microphones on the basis of the difference between the phases of plane sound waves input into these microphones so that a sensitivity has a peak in a direction of a talker and a dip in a noise arrival direction.
In a zoom microphone apparatus disclosed in Japanese Patent No. 4138290, a sound pickup unit converts a sound wave into a speech signal. A zoom control unit outputs a zoom position signal corresponding to a zoom position. A directivity control unit changes the directivity characteristic of the zoom microphone apparatus on the basis of the zoom position signal. An estimation unit estimates the frequency component of background noise included in the speech signal converted by the sound pickup unit. On the basis of a result of the estimation performed by the estimation unit, a noise suppression unit adjusts the amount of suppression in accordance with the zoom position signal and suppresses the background noise. At the time of telescopic operation, the directivity control unit changes the directivity characteristic so that target sound is emphasized, and the amount of suppression of background noise included in a speech signal is larger than that at the time of wide-angle operation.